The present invention relates to an infrared detecting device and a manufacturing method thereof, and particularly to wiring structures of beam portions in a thermal-type infrared detecting device structure.
A conventional infrared detecting element or device has made use of band-to-band transition of electrons with infrared absorption and has been called “quantum type”. In order to improve its detection sensitivity, there was a need to cool it to liquid nitrogen temperature or less and operate it in a state in which thermal noise has been eliminated. Therefore, the quantum type infrared detecting device essentially needs to have a chiller such as a stirling cooler. This is large in size and very expensive. Since its maintenance is also cumbersome in addition to the above, the infrared detecting device has been centered on applications to the field of military technology except for research work.
Since the market publication of an entirely new type infrared detecting device called “non-cooled type (thermal-type)” in the form of its opening to the general public of the military technology, the thermal-type infrared detecting device that makes it unnecessary the chiller has been studied and developed actively. The principle of operation of the thermal-type infrared detecting device is to cause an infrared detection portion thereof to absorb infrared radiation and transform it to heat and allow a thermoelectric transducing portion to detect a change in its temperature.
Several types have been reported as a system for converting the change in temperature to an electric signal. There have been well known a resistor bolometer system for detecting a change in the temperature of a detection portion as a change in electric resistance, a pyroelectric system for detecting a change in spontaneous polarization generated by distortion of a crystal lattice with a change in temperature, a diode system for detecting the dependence of current/voltage characteristics of a silicon pn diode on the temperature, a thermopile system for detecting an electromotive force generated according to the difference in temperature between contacts at two spots, etc.
Since the change in the temperature of the infrared detection portion of the infrared detecting device is converted into the electric signal as the characteristics common to all thermal-type infrared detecting devices, some contrivance to prevent heat or thermal energy based on absorbed infrared rays from escaping to a substrate located around the infrared detection portion is essential for keeping of sensitivity and its improvement.
Assuming now that infrared power incident on the infrared detection portion is Pin, radiation power emitted from the infrared detection portion is Pout, the temperature of the infrared detection portion is Td, the temperature of the substrate is Tsub, and thermal conductance between the infrared detection portion and the substrate is G, the outflow of the incident power from the infrared detection portion to the substrate is proportional to the difference in temperature therebetween and given as G(Td−Tsub) . . . (coefficient/equation 1). Thus, Pin=Pout+G(Td−Tsub) . . . (equation 2) is established from the relation of the conservation law. The flow of heat flux occurs until the temperature of the infrared detection portion and the substrate temperature become equal (Td=Tsub). Assuming now that when the incident power is changed by ΔPin, the temperature of the infrared detection portion is changed by ΔTd, Pin+ΔPin=Pout+ΔPout+G(Td+ΔTd−Tsub) . . . (equation 3) is established. Assuming that ΔPout<<GΔTd, the relationship of ΔTd=ΔPin/G . . . (equation 4) is obtained from the equations (2) and (3). Namely, the amount of change in the temperature of the infrared detection portion is inversely proportional to the thermal conductance G between the infrared detection portion and the substrate.
Namely, the thermal separation between the infrared detection portion and the substrate is essentially required to improve the sensitivity of the thermal-type infrared detecting device. It is very important to reduce the thermal conductance between the two.
In order to realize the thermal separation, there is provided, as a thermal-type common basic structure, a diaphragm structure wherein as shown in FIGS. 1 and 2, an air gap 200 is provided at an interface between an infrared detection portion 140 and a substrate 1, and support legs (called also “beam portions”) 21 and 22 including metal wirings 31 and 32 which support the infrared detection portion from the substrate over the air gap 200 and at the same time take out electric signals of the infrared detection portion to the substrate side are provided (refer to patent documents 1 (Republished Patent Publication No. WO99/31471 (Japanese Patent Application No. 2000-539325)) and 2 (Japanese Patent Application Laid-Open No. 2001-267542), and a non-patent document 1 (SPIE Conference on Infrared Technology and Applications XXV. vol 3698, P556-564, 1999)).
Lengthening the beam portions and thinning same are considered as a choice of the beam-portion structure for reducing the thermal conductance as described above. When each of the beam portions is lengthened, such a structure that the beam portions 21 and 22 shown in FIG. 1 are folded back is taken and the area of the infrared detection portion 140 must be reduced. Therefore, the method of thinning the beam portions 21 and 22 is more effective to reduce the thermal conductance while the area of the infrared detection portion 140 is being maintained. According to the patent document 1 and the non-patent document 1, however, a technical problem arises in that the beam portions cannot be made so thin as will be described below.
FIG. 3 shows a sectional structure (developed section of a spot cut along line X-Y of FIG. 1) of a beam portion 21 analogized from the descriptions of the patent document 1 and the non-patent document 1. In the technology disclosed in the patent document 1, an SOI (Silicon On Insulator) semiconductor substrate 1 is used and a metal wiring 335 is formed over its corresponding silicon film 310 of a portion to be configured as a metal wiring patterned from the silicon film 310 located on an embedded silicon oxide film 300 embedded within the substrate 1, via a metal silicide 325 interposed therebetween. A wiring corresponding to a laminated body of the silicon film 310, the metal silicide 325 and the embedded silicon oxide film 300 is covered over its entirety by protective films of SiO2 or the like such as an interlayer insulating film 340, a silicon oxide film 320 and the embedded silicon oxide film 300, so that the beam portion 21 is configured.
According to the manufacturing method of the patent document 1, the beam portion 21 is formed as described in FIGS. 4 through 9 to be shown below.
As shown in FIG. 4, a silicon monocrystal layer of a topmost surface of an SOI substrate 1 is oxidized such that a silicon film 310 remains corresponding to each desired metal wiring pattern, thereby to form a silicon oxide film SiO2 as a separation film.
Next, a silicon oxide film 320 is formed on each of the silicon film 310 and the silicon oxide film SiO2. Thereafter, each opening corresponding to the desired metal wiring pattern is defined in the silicon oxide film 320 by using a photolithography/etching technique as shown in FIG. 5 thereby to expose the surface of the silicon film 310 (pattern processing of each portion to form silicide.
Next, a metal film to be silicidized is formed on the surface of the exposed silicon film 310 and the silicon oxide film 320, followed by formation of a metal silicide by a quick heating method or the like. The unreacted metal film brought into no silicidization is removed with nitrohydrochloric acid, thereby forming a metal silicide 325 shown in FIG. 6 defined by each opening of the silicon oxide film 320.
Further, a predetermined wiring metal film is thereafter formed on each of the silicon oxide film 320 and the exposed metal silicide 325. Afterwards, the metal film is processed using the photolithography/etching technique, thereby forming a metal wiring 335 corresponding to each desired metal wiring pattern shown in FIG. 7.
Thereafter, as shown in FIG. 8, an interlayer insulating film 340 such as silicon oxide is formed on each of the silicon oxide film 320 and the metal wiring 335 as a protective layer.
Afterwards, as shown in FIG. 9, an etching window Wd for supplying an etchant upon execution of silicon etching is formed so as to extend through the interlayer insulating film 340, the silicon oxide film 320 and the embedded silicon oxide film 300. A gap between the infrared detection portion and the substrate is defined by the etching window Wd. A resist exposure process for forming the etching window Wd generally needs strict mask alignment accuracy.
Finally, a photolithography/etching process including silicon etching conducted via the etching window Wd is done to make the shapes of beam portions 21 and 22, whereby a structure for the infrared detection portion and the substrate both separated from each other by the air gap 200 shown in FIG. 3 is completed. The process steps taken up to here need the photolithography/etching process three times.
Assuming now that as shown in FIG. 10, the width of the beam portion 21 is L1, the width of the metal silicide 325 is L2 and the width (opening) of a forming region for causing a silicide reaction is L3, the relationship of L1>L2=L3 is established. That is, it is necessary to design the width L1 wider than the silicide region width L3 (=L2) so as to be twice (both sides) or more an alignment error in the photolitho process. This is because when the width of the beam portion is not greater than it, the end thereof is in danger of reaching the silicide region upon beam-portion shape etching, and when the silicide region is eliminated by etching, variations in wiring resistance occur, thereby significantly degrading the performance of the infrared detecting device. Further, since the metal wiring is formed on the metal silicide in the case of the patents for reference, the width of the beam portion must be twice or more as wide as an alignment error, for example in addition to the width of this metal wiring.
Assuming that in the case of design using a design rule of about 0.35 μm by way of example, a silicide width is 0.8 μm and a one-side alignment error is 0.3 μm, the metal wiring width and the beam portion width can be thinned only to 1.4 μm and 2.0 μm respectively. In other words, it is considered that when the beam portion width is fixed to a given value L, the silicide width must be set to L2×0.3 or less and hence the related art has required a reduction in resistance by addition of the metal wiring in terms of the electric resistance.
Thus, since the metal silicide is formed within each beam portion and the wiring layer is formed on the metal silicide, a beam-portion width margin for allowing for alignment displacement between those must be ensured, and the narrowing of the width of each beam portion was difficult. Since the wiring layer and the metal silicide are formed as for the wiring on each beam portion, the number of steps was increased in the related art.